The present invention relates to a process and system for producing a fiber suspension from waste paper containing coarse and fine impurities.
In the pulp and paper making industry use of recycled fibers obtained from waste paper is nowadays practised in many paper mills. A key component in the process of producing recycled fibers is the pulper. Bales of waste paper and water are supplied continuously or in batches to the pulper, in which the waste paper is defibrated by a rotating impeller (also called slushing). A so-called extraction plate, which is a special type of screen plate, co-operates with the impeller to create shearing forces on paper fragments to aid in the defibration thereof. Relatively coarse impurities in the waste paper, such as scraps, metal and plastic foils and the like, are retained by the extraction plate, whereas a created fiber suspension containing relatively fine impurities, such as printing ink, coating components, plastic particles, stickies (glue particles) and the like may pass the extraction plate for further processing, i.e removal of fine impurities.
A problem with recycled fibers is that their quality decreases each time they are recycled, because of the mechanical treatment of the fibers by process equipment, such as pumps, screens and refiners. With increasing use of recycled fibers the share of previously recycled fibers contained in waste papers increases. In consequence, it is being more and more difficult to fulfil the uniformity requirement on paper and paperboard produced from recycled fibers thus including increasing amounts of fibers of poor quality. Besides, over time the uniformity requirements tend to continuously be tightening up, which makes it even more difficult to fulfil them.
The necessary mechanical treatment of the waste paper in order to achieve defibration gives rise to another problem, namely that large amounts of extremely short fibers, so-called fines, are created. A great share of fines in a fiber suspension used for producing paper in a paper machine is disadvantageous for many reasons. Firstly, the water permeability of the pulp layer formed in the paper machine will be poor causing decreased capacity. Secondly, the costs for drying the paper will be higher. Thirdly, a large amount of fines distributed in the produced paper results in poorer strength properties of the paper.
A most important paper quality property is cleanliness. To achieve good cleanliness different types of impurities have to be eliminated from the suspension of recycled fibers, which requires advanced cleaning equipment. The cleaning equipment currently in use typically includes several separate interconnected cleaning stations in which the fiber suspension is cleaned in successive stages. However, a problem with this current cleaning equipment is that more fines are created in every cleaning station because of the mechanical treatment of the fibers therein. Besides, some impurities are also disintegrated into smaller particles by the mechanical treatment. This requires installation of further cleaning stations, typically fine screen apparatus as final cleaning stations, capable of separating such smaller particles. Consequently, the traditional system for producing useful clean fiber suspensions from waste paper is complex, relatively expensive and requires a large energy supply for its operation.